Known fluorinated ionomers include copolymers in which sulfonic acid groups or carboxyl groups are bonded to a perfluorinated polymer chain, such as Nafion (trademark), Flemion (trademark), etc. These fluorinated ionomers have been developed as ion-exchange membranes mainly used for brine electrolysis. Researchers are studying their uses as chemical sensors, separation membranes, polymeric superacid catalysts and proton-conducting polyelectrolytes for fuel cells, among others.
Generally, these copolymers are produced by an emulsion polymerization process comprising copolymerizing a sulfonyl fluoride (SO2F) group-containing monomer with tetrafluoroethylene in water containing a surfactant (Japanese Unexamined Patent Publication No. 1987-288614, etc.); a solution polymerization process carried out in a solvent compatible with sulfonyl fluoride group-containing monomers; a suspension polymerization process carried out in a dispersion of the solvent in water, or like process. The polymerization medium used in these polymerization processes is an organic solvent compatible with sulfonyl fluoride group-containing monomers, generally a fluorine-containing solvent. For example, Japanese Unexamined Patent Publication No. 1994-199958 proposes a technique using a hydrofluorocarbon, and Japanese Unexamined Patent Publication No. 1994-19959 proposes a technique using a hetero atom-containing perfluorocarbon.
Another process proposed for producing a copolymer having sulfonate functional groups bonded to a perfluoropolymer chain is a technique using a monomer containing sulfonate functional groups (SO3M; wherein M is an alkali metal). For example, U.S. Pat. No. 3,560,568 states that a white solid copolymer was obtained by ordinary emulsion copolymerization of CF2═CF—O—CF2CF2—SO3Na with tetrafluoroethylene in water containing a surfactant dissolved therein.
However, the present inventors' investigation showed that, when a sulfonate functional group-containing vinyl ether monomer was copolymerized with tetrafluoroethylene in water, the amount of the vinyl ether monomer introduced was extremely small (not more than 1 mole %), and the obtained polymer was mostly a homopolymer of tetrafluoroethylene (PTFE). This is presumably because the sulfonate functional group-containing vinyl ether monomer is water-soluble whereas tetrafluoroethylene is hardly water-soluble, so that it is difficult for the vinyl ether monomer and tetrafluoroethylene to coexist in the place of polymerization (in this case, the vicinity of a micelle formed by the surfactant).
Further, WO98/43952, pamphlet, discloses that another process called an iodine-transfer polymerization process gives a copolymer of a monomer represented by Formula (1′)
wherein M is an alkali metal or an alkaline earth metal, and n is 0, 1 or 2, with another monomer. This process is capable of producing a copolymer to which the monomer of Formula (1′) has been introduced at a desired ratio.
However, the process has the problems that it involves relatively complicated manipulation and necessitates the use of an expensive iodine compound.
Moreover, in the case where tetrafluoroethylene is used as the other monomer, the obtained aqueous dispersion of the copolymer is instable especially when the amount of the monomer of Formula (1′) is relatively small. In particular when synthesized in a large amount, the obtained copolymer is difficult to purify.
A polymer is purified generally by freezing a polymer dispersion or adding an electrolyte or an acid to the polymer dispersion to coagulate and separate the polymer, followed by washing, extraction or like procedure.
However, fluorinated ionomers, especially those containing sulfonate groups, are stable as solutions or dispersions, and thus are difficult to purify by the known method.
Further, a film of a fluorinated ionomer can be formed by casting, but the film may crack during drying, depending on such factors as the type of material, the film thickness, etc.
An object of the present invention is to provide a process for producing a fluorinated ionomer that can be easily purified.
Another object of the invention is to provide a method for purifying and concentrating a fluorinated ionomer, and a method for forming a film free of cracking.
Another object of the present invention is to provide an industrially easy process for producing a fluorinated ionomer having sulfonic acid functional groups bonded to a perfluoropolymer chain, using a sulfonic acid functional group-containing perhalogenated monomer.